Abstract
We investigated the roles of one-photon and two-photon processes in the laser-controlled rovibrational transitions of the diatomic alkali halide, 7Li37Cl. Optimal control theory calculations were carried out using the Hamiltonian, including both the one-photon and two-photon field-molecule interaction terms. Time-dependent wave packet propagation was performed with both the radial and angular motions being treated quantum mechanically. The targeted processes were pure rotational and vibrational–rotational excitations: (v = 0, J = 0) → (v = 0, J = 2); (v = 0, J = 0) → (v = 1, J = 2). Total time of the control pulse was set to 2,000,000 atomic units (48.4 ps). In each control excitation process, weak and strong optimal fields were obtained by means of giving weak and strong field amplitudes, respectively, to the initial guess for the optimal field. It was found that when the field is weak, the control mechanism is dominated exclusively by a one-photon process, as expected, in both the targeted processes. When the field is strong, we obtained two kinds of optimal fields, one causing two-photon absorption and the other causing a Raman process. It was revealed, however, that the mechanisms for strong fields are not simply characterized by one process but rather by multiple one- and two-photon processes. It was also found that in the rotational excitation, (v = 0, J = 0) → (v = 0, J = 2), the roles of one- and two-photon processes are relatively distinct but in the vibrational–rotational excitation, (v = 0, J = 0) → (v = 1, J = 2), these roles are ambiguous and the cooperative effect associated with these two processes is quite large.
Highlights
Recent rapid developments in laser technology have prompted a huge number of studies aiming at a better understanding of very fast dynamical events behind physical, chemical, and biological phenomena
It is not until the developments of ultrashort pulse generation and the pulse shaping technique that one can use the quantum control idea [1,2,3,4] to achieve the goal of relevant physical processes
We [30] recently proposed a control scenario for the isotope-selective rovibrational excitation of diatomic molecules at a finite temperature; first a frequency comb for rotational excitations is applied to change the rotational-state distributions and to magnify the difference in vibrational transition energies between the isotopologues, and second, a bunch of pulse is irradiated for isotope-selective vibrational excitations
Summary
Recent rapid developments in laser technology have prompted a huge number of studies aiming at a better understanding of very fast dynamical events behind physical, chemical, and biological phenomena. We [30] recently proposed a control scenario for the isotope-selective rovibrational excitation of diatomic molecules at a finite temperature; first a frequency comb for rotational excitations is applied to change the rotational-state distributions and to magnify the difference in vibrational transition energies between the isotopologues, and second, a bunch of pulse is irradiated for isotope-selective vibrational excitations. It was shown in the numerical simulation that the proposed scheme works well for the isotope-selective rovibrational excitation for a gas-phase mixture of the diatomic alkali-halide 7 Li37 Cl and 7 Li35 Cl molecules at 70 K.
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